Ballistic pendulum
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Deffinition- A physical pendulum consisting of a large mass suspended from a rod and a bullet is shot into it.

What it is used for: when it is stuck by a projectile its displacement is used to measure the projectile's velocity.


As seen from these equations you can figure out the velocity or mass of the bullet from the height. It goes the same for the block of mass. The information that we always have is the height.

The main principles used in this are Conservation of momentum, Conservation of energy, and gravitational potential energy is conservative.

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Here is a set up of a ballistic pendulum
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As shown from this picture. physics is lame. or maybe that girl is just lame...

Sources:
http://hyperphysics.phy-astr.gsu.edu/hbase/balpen.html
http://www.thefreedictionary.com/ballistic+pendulum